Controllers for operating a centrifugal compressor along a path of minimum power consumption typically reduce the speed of the compressor in response to a lower capacity demand. But when the speed drops too low, an undesirable surge condition can occur. When this happens the discharge pressure and flow rate fluctuate, and in severe cases, the flow can actually reverse direction. The poor performance caused by surging can also be accompanied by excess noise and damaging vibration.
In attempts to solve this problem several controllers have been developed which avoid operating compressors in the surge region. For example, U.S. Pat. No. 4,581,900 describes a surge detection method based both upon the rate of change of pressure developed across the compressor and the rate of change of current passing through the compressor motor. This detection method, as described in the patent, also includes selecting a pre-specified reset time interval based upon the size of the compressor. Although this patent offers a method of detecting a surge, which is not difficult to do since surges are typically quite noticeable and often violent, it offers no means of distinguishing a subtle impending surge condition from normal operating conditions.
In another example, U.S. Pat. No. 4,177,649 discloses a controller for a centrifugal compressor which identifies a surge condition as a reduction in discharge flow with a corresponding reduction in discharge pressure. In a graphical illustration representing discharge pressure on the vertical axis and flow rate on the horizontal axis, this patent defines the surge region as the portion of the graphed function having a positive slope. Conversely, the non-surge region is the portion of the graphed function which has a negative slope, wherein the flow increases as the discharge pressure decreases. The controller continuously varies the capacity of the compressor alternately about an operating point and senses two parameters, e.g., discharge pressure and discharge flow, that not only vary with the capacity but also are used to determine the slope of the function. Surge is then avoided by operating only in the negative slope region. This controller may be useful in some applications, but problems are likely to arise if it is used with compressors having a load curve with a substantial flat region between the positive and negative slope portion of the curve so that the point of impending surge is not clearly defined. Worse yet, this controller would not be effective with many compressors which have a surge region that extends into the negative slope portion of the curve.
Two other controllers which avoid the problems and limitations of the slope sensing method are disclosed in U.S. Pat. Nos. 4,151,725 and 4,546,618. These two controllers include a microprocessor with access to data stored in memory defining the operating curve and/or surge region of the specific compressor it is controlling. Unfortunately, these controllers are limited to controlling only compressors having a particular load curve that corresponds to the stored data. Before the microprocessor program can be used, the load characteristics of the particular compressor model must be known and these characteristics may vary between compressors of the same design. In addition, the load characteristics can be affected not only by the system associated with the compressor but also by aging of both the compressor and its system.
U.S. Pat. No. 4,355,948, Kountz et al, discloses a controller which is essentially the same as the '725 Kountz et al, but further includes two field adjustable potentiometers for custom fitting the operating curve to match the load conditions of a particular compressor in a particular application. The disadvantage of this controller is that it is not universally adaptable to a variety of compressors without first preprogramming an operating curve to match the specific compressor and later field adjusting it to match the system.
U.S. Pat. No. 3,778,695 discloses a controller for a motor driven centrifugal compressor that claims to identify the onset of a surge by measuring the motor current. When the current drops below a predetermined value, it is assumed that a surge is impending and a relief valve is opened. One problem with this method is that the current level at which a surge is impending is not a constant. It varies as a function of compressor load, which in turn, can vary from one compressor to the next and from one system to the next. Although this method may be useful as a lower limit safety device, it would not enable operation of a compressor along its path of minimum power consumption.
In consideration of the above, it is an object of this invention to provide a controller that operates a centrifugal compressor along its path of minimum power consumption while avoiding a surge condition.
Another object is to provide a controller that is universally adaptable to a variety of compressors without having to preprogram an operating curve to meet the characteristics of a specific compressor.
Another object is to provide a controller that senses the onset of a surge over a wide range of operating points, including the negative slope portion of the pressure/flow curve.
These and other objects will be apparent from the attached drawings and the description of the preferred embodiments that follow below.